As it is known, among the most important objects in the energy and environmental fields there are undoubtedly the reduction of the concentration of greenhouse gases in the atmosphere and the storage thereof. In this regard, of particular interest is the separation of carbon dioxide (CO2) from gases introduced in the atmosphere, such as, for example, industrial fumes and combustion gases.
Furthermore, carbon dioxide is present in natural gas, biogas, and other gaseous mixtures of industrial use. In this case also, is carbon dioxide separation is required in order to improve the quality specifications of gas for industrial use.
Different technologies for separating CO2 from a gaseous mixture are present in the commerce. The selection of the technology to be used depends on the purity required for the product, and on the conditions of the gas to be treated (temperature, pressure, impurities present or concentration of CO2 in the gas).
However, the high costs and the energy required by the currently available processes which are based on such known technologies represent the main obstacles to the actual use thereof. Typical applications are the purification of industrial fumes, natural gas (methane) or biogas, while the direct capture of carbon dioxide from the atmosphere still remains a far target, due to the huge volumes of air to be treated (due to the low concentration of CO2 in the air, below 400 ppm [http://co2now.org/]).
The most widespread types of purification plants for CO2-rich gases are essentially attributable to techniques which use:                gas-selective membranes,        low-temperature methanol,        water at variable pressure, or        
basic solutions of ammonia or amines, free or absorbed onto solid supports.
Each of these techniques has drawbacks, such as investment or maintenance cost, problems of environmental dispersion, use of high amounts of water, corrosion processes of the plant.
More in detail, as regards to the technology of CO2 separation by using a gas-selective membrane, as described for example in U.S. Pat. No. 8,052,776, it shall be noted that such technology does not ensure, compared to the chemical absorption, a good quality of the extracted gas, and it requires a gas in inlet with a low partial pressure of CO2. On the other hand, the use of membranes having a high efficiency, as described for example in U.S. Pat. No. 8,506,677, requires a high investment cost, in addition to multiple extraction steps to achieve an acceptable separation. Furthermore, the membranes can easily obstruct, due to the micro-particles dragged by the gas passing through such membranes. This requires frequent interruptions of the process for cleaning the pores, with additional operative costs.
A known technology, alternative to the use of gas-selective membranes, is based on chemical processes of liquid-phase carbon dioxide absorption, which mainly exploit the acid feature of carbon dioxide. The absorbent molecules can be some organic amines, as described for example in EP 2514509 and WO 2012142668, or alkali metal hydroxides, as described for example in U.S. Pat. No. 8,119,091. In the case of organic amines, the main problem relates to the environmental dispersion. Simple amines are typically volatile, with a medium-high toxicity. This necessarily leads to configure the plant so as to ensure a perfect confinement of the vapors. The reaction between carbon dioxide and amines leads to the formation of http://co2now.org, which, by providing heat, releases CO2, thus regenerating the starting amine. The thermal energy necessary for the release is related to the molecular structure of the amines. For the simple amines, the temperature required for the decarboxylation reaction is near to the boiling temperature of the free molecule, and this complicates the problems of vapor confinement.
The alkali hydroxides are safer as regards the environmental dispersion, but they have a high basicity, resulting in corrosion problems. Therefore, the hydroxides have to be used in more or less diluted aqueous solutions, and this involves an excessive absorption of thermal energy in the releasing steps. The reaction product of carbon dioxide with an inorganic hydroxide is bicarbonate. Sodium bicarbonate, for example, has a solubility in water of about 100 g/L. Therefore, during the absorption step, the precipitation of bicarbonate micro-crystals inside the reactor is possible.
Recently, the ionic liquids for the continuous absorption of carbon dioxide have been introduced. These are liquid-phase organic salts, and some of them have a specific reactivity with carbon dioxide via the formation of carbamic acid or by a dipolar interaction. See, for example, the ionic liquids described in US 20120186993, U.S. Pat. No. 7,527,775 and WO 2012033991. The ionic liquids have been mainly used in the gas-selective membranes, as described for example in US 20130225401, in the form of a polymer, as described, for example, in WO 2006026064, or via electrospray devices, as described for example in U.S. Pat. No. 8,480,787. However, the known ionic liquids used for the capture of carbon dioxide are typically complex molecules, so that their synthesis turns out to be expensive, as well as their not very interesting use for an industrial application. Furthermore, many ionic liquids have high viscosities, which limit the efficiency thereof.
As the gas-selective membranes having amines as the absorbing molecules, also the ionic liquids can be cooled by promoting the release of CO2 previously absorbed by carbamic acid. However, such process requires a considerable energy consumption. It follows that the stripping process requires the introduction of heat to promote the cleavage of the bond between the carbonyl carbon and the nitrogen of the carbamate. Inversely, in the absorption step, the carbamate formation reaction produces heat which, if it is not subtracted from the system by a suitable cooling system, reduces the efficiency of the reaction.
In this context, systems exploiting the combination of a basic organic molecule capable of absorbing CO2 with an apolar solvent are also known. In fact, the presence of apolar molecules in admixture with some types of absorbing molecules promotes the release of carbon dioxide. This phenomenon is due to the change in the polarity of the absorber molecule bound to CO2 with respect to the free state, i.e., the initial state in the original form not bound to CO2. In particular, carbamate which is formed during the absorption step is more polar compared to the molecule in its initial state, i.e., with a free amine group. Therefore, the CO2 releasing step is promoted if in the absorbent mixture there is an apolar solvent, also called anti-solvent, due to a higher affinity towards the apolar solvent of the low polarity molecule which is reforming. This phenomenon is known as polarity swing assisted process (PSAR), and it is disclosed also in U.S. patent 2013/0056676. However, such system provides for the use exactly of an apolar solvent (anti-solvent), which is a non-biodegradable hydrocarbon. Furthermore, the anti-solvent hardly has ideal characteristics of miscibility with the absorbent species, necessary to ensure a process of absorption efficient that is efficient and, at the same time, a low vapor pressure and a high self-ignition temperature, both of which are undesired conditions due to obvious environmental and plant safety reasons.
Therefore, it is the main object of the present invention to provide a method for the separation of carbon dioxide from a gaseous mixture by using an liquid-phase absorber which has a high efficiency and a low environmental impact, while having an easy and cost-effective production for an industrial application, so as to overcome the limits of the above-mentioned known technologies for capturing carbon dioxide.
Another object of the present invention is to provide a method for the release of carbon dioxide from the liquid-phase absorber used for the capture thereof, which has a high efficiency, a low environmental impact and which requires reduced operative temperatures.
Still a further object of the present invention is to provide a plant capable of optimizing the heat exchange between the absorption step and the releasing step so as to minimize the energy intake.